A thermal flowmeter is used to measure a flow amount of a gas and includes a flow measurement element which measures a flow amount. Here, the flow amount of the gas is measured in such a manner that heat is transmitted between the flow measurement element and the gas which is a measurement object. The flow amount which is measured by the thermal flowmeter is widely used as an important control parameter of various devices. The thermal flowmeter has a feature that a gas flow amount, for example, a mass flow amount can be measured with relatively high accuracy compared to other flowmeters.
However, there has been a desire to further improve measurement accuracy of a gas flow amount. For example, in a vehicle equipped with an internal combustion engine, there is an extremely high demand of saving fuel or purifying an exhaust gas. In order to handle these demands, an intake air amount which is an important parameter of the internal combustion engine needs to be measured with high accuracy.
A thermal flowmeter which measures an amount of intake air led to an internal combustion engine includes a sub-passage which takes a part of the intake air amount and a flow measurement element which is disposed in the sub-passage. Here, a state of a measurement object gas flowing in the sub-passage is measured by the transmission of heat between the flow measurement element and the measurement object gas and an electric signal indicating an amount of the intake air led to the internal combustion engine is output.
For example, as a technology of such a thermal flowmeter, PTL 1 discloses a “flow amount measurement device in which a plate-shaped board is disposed to form a fluid passage at each of a sensor element mounting face in the plate-shaped board and a rear face opposite to the sensor element mounting face and a curve passage portion is formed at an upstream side of the plate-shaped board of a sub-passage so that a direction changes.” In this Patent Literature, a “wall face in the vicinity of the curve passage portion is provided with an inclined portion which is inclined so that an end located near a side wall face of the curve passage portion facing the sensor element mounting face is located at an inner loop of the curve passage portion in a direction following the side wall face.”